The use of semipermeable membranes for the separation of gases or liquids in reverse osmosis or ultrafiltration processes is well known. For example, in a reverse osmosis process, high pressure saline water may be placed in contact with a semipermeable membrane which is permeable to water but relatively impermeable to salt. Concentrated brine and relatively pure water are separated thereby; the relatively pure water may then be utilized for personal use such as drinking, cooking, etc., while the brine may be discarded. In addition, membranes may also be utilized for the separation of various gases. The separation of a gas mixture utilizing a membrane is effected by passing a feed stream of the gas across the surface of the membrane. Inasmuch as the feed stream is at an elevated pressure relative to the effluent stream, a more permeable component of the mixture will pass through the membrane at a more rapid rate than will a less permeable component. Therefore, the permeate stream which passes through the membrane is enriched in the more permeable component while, conversely, the residue stream is enriched in the less permeable component of the feed.
This ability to separate gases from a mixture stream will find many applications in commercial uses. For example, gas separation systems may be used for oxygen enrichment of air, for improved combustion efficiencies and conservation of energy resources. Likewise, nitrogen enrichment of air may be applicable where inert atmospheres are required. Other applications for oxygen enriched gases may be improving selectivity and efficiency of chemical and metallurgical processes. Similarly, inert atmospheres such as may be provided for by this invention may also be utilized in chemical and metallurgical processes. Some other applications of gas separation would include helium recovery from natural gas, hydrogen enrichment in industrial process applications, and scrubbing of acid gases. Specific uses for oxygen enrichment of air would be breathing systems for submarines and other underwater stations, improved heart-lung machines, and other lung assist devices. Another specific application of a gas separation system would be an aircraft to provide oxygen enrichment for life-support systems and nitrogen enrichment for providing an inert atmosphere for fuel systems. In addition, gas separation systems may be used for environmental benefits, e.g., methane can be separated from carbon dioxide in waste gases for sewage treatment processes and oxygen enriched air can be produced to enhance sewage digestion.
Another use for which membranes may be employed is the separation of polysaccharides into useable constituents. For example, in many commercial enterprises sugar is utilized to a great extent for its sweetening properties. It is used in the sweetening of foods, for the manufacture of syrups and confectionery items, in preserves and jams, as a chemical intermediate for detergents, emulsifying agents and other sucrose derivatives such as plasticizers, resins, glues, etc. The usual derivation of sugar is from cane sugar and sugar beets. It is obtained by crushing and extracting the sugar from the cane with water or extracting the sugar from the sugar beet with water followed by evaporation and purifying with lime, absorbent carbon and/or various liquids. The chief component of this type of sugar is sucrose, while other sugars may contain components such as dextrose, glucose and fructose and other polysaccharides. Other polysaccharides which possess sweetening properties include maltose, etc. The various polysaccharides possess varying degrees of sweetness, especially when in pure form and not contaminated by any reversion products.
One source of glucose which possesses a relatively high degree of sweetness and which, in turn, may be converted to fructose, the latter possessing an even greater degree of sweetness is starch. As is well known, starch is present in many naturally-occurring plants including corn, potatoes, rice tapioca, wheat, etc. Heretofore, it has been known to treat starch with an enzyme such as amyloglucosidase to obtain glucose. However, the treatment heretofore provided entailed a relatively long residence time in order to obtain a glucose syrup which contained about 94% glucose. The relatively long residence time which has heretofore been required restricts the throughput of glucose and results in the appearance of reversion products which impart a bitter taste to the glucose, thus negating the sweetening property of the compound as well as requiring further treatment in order to remove the offending product. One such reversion product which imparts a bitter taste comprises isomaltose.
Many methods involving the use of an enzyme such as amyloglucosidase to convert starch into sugar have been tried. However, each of these methods has some disadvantages attached hereto. For example, when using a free enzyme, it is necessary to continuously replace the enzyme which is lost during the production of the desired saccharide. Likewise, when using an immobilized enzyme, the heretofore relatively long residence time has resulted in the appearance of unwanted side products.
One method of overcoming many of the disadvantages hereinbefore set forth is to contact the feedstock such as starch with an enzyme for relatively short residence time and thereafter subjecting the partially hydrolyzed reaction mixture which is obtained from the conversion reaction to an ultrafiltration step wherein said reaction mixture is passed over a membrane whereby higher glucose syrup will pass through the membrane as a permeate while the retentate material containing unhydrolyzed oligosaccharides may be recycled for additional treatment.
In addition other uses of membranes will include milk whey separation as well as concentration of proteins.
As will hereinafter be shown in greater detail, by utilizing the membranes of the present invention, it is possible to obtain a high degree of saccharide separation, which results in the obtention of desired products at a relatively low operating cost. It is also possible to obtain a high degree of separation of amino acids from oligopeptides and polypeptides, the "polymers" of amino acids. Oligopeptides generally include the shorter chain polymers of a given amino acid, e.g. the dimers, trimers, tetramers, and possibly pentamers of amino acids. Polypeptides are the larger polymers.
Heretofore, membranes which may be used for reverse osmosis or ultrafiltration processes have been prepared using a wide variety of chemical compounds to obtain the desired membrane. For example, U.S. Pat. No. 3,892,655 discloses a membrane and a method for producing the membrane in which a thin polymer film is formed on the surface of a liquid, generally water and is subsequently transferred to the surface of a porous supporting membrane. During the transfer of thin polymer film, the porous support is maintained in a wetted stage with the liquid. Another U.S. Pat. No. 3,516,588 discloses a macromolecular fractionation process and describes a porous ultrafiltration membrane which is selective on the basis of pore size. Likewise, U.S. Pat. No. 3,767,737 discloses a method for producing the casting of "ultra-thin" polymer membranes similar in nature to previously mentioned U.S. Pat. No. 3,892,655 in that the thin film of the membrane is formed on the surface of a liquid and transferred to the surface of a porous support membrane. However, the thin film polymer will thus inherently possess the same disadvantage which may be ascribed to the membrane formed by the latter patent in that the thin film of the finished membrane is weakly attached to the porous support and the membrane thus produced cannot withstand substantial back pressure when in operation.
As was previously mentioned, semipermeable membranes have been prepared from a variety of compounds by utilizing a polymer as the membrane-forming material. Examples of semipermeable membrane-forming polymers which have been used will include silicon-containing compounds such as dimethyl silicone, silicone-carbonate copolymers fluorinated silicones, etc., polystyrene-polycarbonate, polyurethanes, styrene-butadiene copolymers, polyarylethers, epoxides, cellulose nitrate, ethyl cellulose, cellulose acetate mixed with other cellulose esters, etc. The membrane resulting from the polymer is usually composited on a finely porous support membrane such as polysulfone, cellulose nitrate-cellulose acetate, etc., the composition being, if so desired, impregnated on a natural fabric such as canvas, cotton, linen, etc. or on a synthetic fabric such as Dacron, Nylon, Orlon, etc.
Examples of some semipermeable membranes which have been used in the past are those described in U.S. Pat. No. 4,005,012 which discloses a thin-film composite membrane comprising a cross-linked epiamine composited on a porous support such as polysulfone, the composition being impregnated on a backing material such as Dacron. U.S. Pat. No. 4,132,824 discloses an ultra-thin film of a polymer composite comprising a blend of a methylpentene polymer and an organopolysiloxane-polycarbonate interpolymer while U.S. Pat. No. 4,243,701 discloses a membrane comprising a dimethyl silicone composited on a cellulose nitrate-cellulose acetate support member.
In addition to the aforementioned patents, other U.S. patents also disclose membrane materials. For example, U. S. Pat. No. 4,262,041 discloses a process for preparing a composite amphoteric ion-exchange membrane by forming a membrane from a solution or emulsion of a polymer having either a cation or anion exchange group and a functional group which is capable of receiving another ion exchange group having an opposite sign from the cation or anion exchange group or a mixture of two polymers, one of which has either a cation or anion exchange group and the other polymer has a functional group capable of receiving an ion exchange group having the opposite sign from the cation or anion exchange group and introducing the ion exchange group of an opposite sign to said functional group. This is in contradistinction to the membrane of the present invention in which a composite of two polymers, said composite already being cross-linked, is cast on a porous support member.
U.S Pat. No. 3,661,634 describes the use of an Interpenetrating Polymer Network membrane for reverse osmosis. The membrane is prepared from poly (vinyl-pyrrolidinone) as the host polymer and polyisocyanates as the guest polymer component with these latter prepolymers being chosen from those materials used in urethane coating and foam applications. The membrane is generated by casting a solution of the host and guest polymer and then curing said membrane via a two-stage process. The result is a membrane which exhibits increased salt rejection with increasing isocyanate equivalent ratio.
U.S. Pat. No. 4,272,378 is drawn to a semipermeable membrane involving the use of polymers containing more than 40 mole percent of acrylonitrile, said acrylonitrile being copolymerized with other monomers. The result is a membrane which will possess characteristics and performances which are entirely different and apart from those which are possessed by the membranes of the present invention. U.S. Pat. No. 4,220,535 claims a multi-zones hollow fiber permeator which may be obtained from any suitable synthetic or natural material suitable for fluid separation or as supports for materials with solutions of polyamide amines, said admixed solution being cast into membranes. In this patent, the matrix materials such as poly (phenyl ether) are intimately admixed with the polymers, this admixture being unlike and distinct from the membranes of the present invention which are hereinafter set forth in greater detail.
Another U.S. Pat. No. 3,549,569 discloses the use of one-component polyurethane coatings or moisture cured coatings. In this patent, the coatings are based on the use of moisture curing 1-isocyanate-3-isocyanatomethyl-3,5,5-trimethylcyclohexane capped polyether (polyols) having a molecular weight of at least 500. The resulting coatings are flexible, impact resistant and mar resistant.
Another type of semipermeable membrane comprises that prepared from a electrolyte complex. For example, Canadian Patent No. 836,342 describes a membrane prepared from a electrolyte polymer which is cast as a porous film on a support. The resulting membrane comprising the porous electrolyte film on the support is anisotropic in nature. In contrast to the membrane of the present invention, it should be noted that both the rejecting layer and the porous layer of the membrane are comprised of the same material, i.e., the electrolyte polymer. Such membranes, when subjected to high pressures during use, tend to undergo compaction which then results in reduced permeate productivity after a period of time. The compaction of the membrane is due to the collapse of the relatively weak porous structure under pressure. The patent further states that, in general, the concentration of each component of the electrolyte polymer must be at least 0.5% by weight and preferably above 5% by weight of the total solution in order to obtain a satisfactory product. This means then that there must be at least 1% by weight and preferably above 10% by weight of both components of the complex in the solution used to obtain the membrane. Furthermore, the patent also states that the overall thickness or gauge of the membrane varies from 1 to about 20 mils of the membrane and from about 1 to about 10 mils in thickness for the support member of the total product.
As will hereinafter be shown in greater detail it has now been discovered that thin film composite ultrafiltration membranes may be prepared from a polyelectrolyte complex composited on a porous support in which the thin film composite comprises a dense selective relatively thin layer composited on a micro porous sub-layer support. The resulting membrane will be superior to prior art ultrafiltration membranes which are anisotropic in nature due to the fact that the membrane of the present invention will be more resistant to pressure compaction as well as possessing better adhesion between the components and thus will be able to perform the intended use for a longer period of time without the necessity of being replaced.